Max Fish's 4 Link Article/Information

Why so much talk about rear suspension? It seems these days no self-respecting minitruck is show worthy without an attention commanding “Goldberg device” (look it up) hanging out of a cavernous hole in the bed. Most rear suspension designs are limited to certain confines that don’t create many severe affects, but there are major design considerations that should be taken in account. Regardless of how you lift the frame off of the rear end, the links that connect the rear end to the chassis are what affect the dynamic relationship of the vehicle to the road. Before we go into the actual suspension designs I think it would benefit us to define the affects.Every suspension design has its benefits, even the tried-and-true 4-link has infinite design options to be an advantage in one area more than the other. The dynamics that are affected by the design, that are important to us here are…

- Roll-center- Roll-steer- Anti-squat- Pinion angle

Also, before we even discuss these we need to define instantaneous center or instant center. What instant center is is a theoretical point in space that is used to calculate suspension affect. The way to find instant center is easy once you “get it.” When you have two or more, bars, arms or what ever you are working with, each part is only as long as a straight line drawn from pivot to pivot. If you were to extend the line drawn through those points into theoretical “space” and do the same through the pivots of the other related parts, hopefully, at some point, they will meet. The point at which they meet is the instant center. If this doesn’t make any sense keep thinking about it until it does. To continue reading without understanding instant center is not going to be as fruitful as it should be.

Now that you know what instant center is let’s talk about roll-center. Every suspension design sets a point at which the vehicle “rolls” from side-to-side. Most of the time the point moves around depending on where the suspension is in it’s travel, but it is still important. On a straight axle type rear suspension the roll-center is “set” by whatever is used to eliminate lateral movement (i.e. panhard bar or even a triangulated type link system.) What happens, is that the center of gravity acts upon the vehicle through its roll-center. The higher the roll-center the lower the vehicle feels in cornering. Don’t get greedy though, too high of a roll-center and the car will act odd. The chances of having a high roll-center at the front suspension is not good and the vehicle will act unbalanced. Any decent rear suspension design will have a roll center around the center of the rear end, which is fine.Roll-steer is a little more important and could cause some really poor dynamic affects. Roll-steer also comes in flavors, roll-understeer and roll-oversteer. Every straight-axle rear suspension design, that actually affects performance, has roll-steer in some amount. Roll-steer is always measured when the car is leaning toward the outside of a corner. For example, if you were to turn left the vehicle would lean to the right, so with the right rear wheel being compressed and the left wheel in droop and geometry being what it is, the rear end would try to steer the vehicle into or out of the corner. The issue is caused when the rear end steers out of the corner causing roll-oversteer. What happens at the controls of the vehicle is once the corner is entered and the vehicle takes its “set” into the corner, the rear end suddenly steers the car further than initially anticipated. Now under normal driving conditions this is at best annoying, but really driving into a corner and having the rear end oversteer can put the car into a spin. In really severe conditions the vehicle would be extremely difficult to drive over 30mph, a small bump in the road would really upset the chassis and almost can’t be controlled. Again the confines of the typical minitruck doesn’t really leave much room for severe oversteer and severe understeer is hard to achieve.Anti-squat is really not a major concern, except that it should be at least taken into consideration while building a link system that is limited to short bars. Anti-squat also affects roll-steer and vise-versa (roll-center can also affect roll-steer, but isn’t a concern with a properly designed system.) What anti-squat does is fight the affect of weight transfer trying to “squat” the rear end while under acceleration. To create or eliminate anti-squat, you would design the link system to have the instant-center a certain height, related to the center of gravity. A higher instant-center has more anti-squat while a lower instant-center has less. At some point too high of an instant-center will actually lift the rear up as the vehicle accelerates. And too low will push the rear down (though a little tougher to achieve.)Pinion angle is probably the one thing that most builders are completely aware of, but many are not sure how to set up properly. The pinion angle should be between 1 and 3 degrees pointed down. Most of us have already heard that, but the thing is that the major detail left out is that it should be 1 to 3 degrees pointed down from parallel to the transmission (or the front driveshaft on a 2 piece drive line.) What that means is the pinion could, under certain circumstances, point up. We don’t have near enough space here for me to explain how to design the suspension that you may desire, but give us some time we will get there.

Now that the boring part is over lets get to the subject at hand… Suspension design. I am going to describe the major benefits of each design. (None of this is my opinion (unless noted.) So don’t go and think that I am just bad-mouthing every body else’s ideas.) For a suspension design to be un-drivable, it would have to be really bad. Also if you just can’t quite fit the perfect design into your confines you will have to compromise. That’s part of what makes a good fabricator, the ability to adapt to their limitations. So here we go…

The ladder-bar and 2-link are both very similar. In fact a ladder bar is a type of 2-link, but not all 2-links could be considered ladder-bars. The ladder-bar design is great for high horse power, high traction cars that the owner doesn’t want to spend the hours or days it would take to tune a good 4-link for the track. The first major downfall of the 2-link is that it doesn’t allow much articulation (one wheel up and one wheel down.) The only way that the design can articulate is to flex a little bit of everything. NASCAR uses a 2-link rear suspension that is specially engineered to allow an acceptable amount flex in order to articulate. The second major issue is pinion angle. The 2-link by nature cannot keep the pinion parallel to the transmission so useable travel is limited to the acceptable amount of pinion change. On the other hand the 2-link is fairly easy to tune, to install, and to purchase. Anti-squat is easy to figure because the instant-center is the forward pivot. Roll-center and roll-steer aren’t really a concern because the suspension won’t articulate so on the same coin the need for a swaybar is just not there either. The 2-link will require a panhard bar, track locator or other similar lateral limiter.

The 4-link is separated into (3) major types

- Triangulated 4-link- Parallel 4-link- 3-link

Each of these designs have the same advantages and can be oriented as forward, reverse or “2-forward 2-reverse.” Initially I am only going to talk about the forward facing orientation. Once we have described the advantages of those, I will explain the downfall of each of the other orientations. The reason that the 3-link is categorized here is that just about every advantage and disadvantage is shared by all three of these systems. In fact the 3-link and parallel 4-link are identical except the missing link. It is a space saver, period.The way that a link system of this type is drawn out is 1 dimensionally, from the side. From that view you cannot tell which design is used. The side view is how you would figure instant center, anti-squat, pinion angle and a potion of roll-steer. Because of the infinite possibilities of the 4-link, they are used on just about every straight axle rear suspension in the performance industry (factory or after market.) Good control of the pinion is possible (assuming the designer took it into consideration,) they also have infinite instant center options. As a matter of fact the drag race community has 4-links designs that allow tuning of the instant center separately from one side to the other. Again the confines and the fear of most builders to “step out of the box,” have limited our designs, but the limitations have been on the side of caution. Having an instant center around the front bumper or beyond has very little adverse effects. This how the manufactures set up their link systems.The separation of each of these designs is from above. From this view you can see how each bar is run. The triangulated 4-link would be the most obvious and is what is most commonly used in the minitruck industry. The triangulated 4-link does not need a panhard bar or any other lateral limiter (the triangulation of the bars does the limiting for us.) This would be a god time to touch on the “wishbone” type 3-link. What this system does is bring the close end of the triangulated into each other, creating the wishbone. It acts exactly like the triangulated 4-link except that the single point of the wishbone is the roll-center. Minitrucks don’t really use this system because of space limitations. Early Cadillacs used this system and had a reputation of breaking the balljoint used at the single point. The overhead view also is another addition to the roll-steer picture. By pointing the bars towards the front or rear of the vehicle can change the roll-steer affect.The 3-link and parallel 4-link need a lateral limiter of some sort, which would be viewed from the rear (in most cases.) A panhard bar, track-locater (viewed from above,) Watts-link… There are many different ways to limit lateral movement and all have their advantages and disadvantages. But that’s for another day…Forward, reverse, what’s the difference? A forward facing 4-link transfers energy to the chassis planting the rearend under power, a small amount of anti-dive under braking and can be tuned infinitely for a specific purpose. A reverse 4-link on the other hand should only be used as a compromise. I know that there are a whole bunch of you that have reverse 4-links and think they work great. I’m sure they do. They will drive fine and turn corners and stop the car without too much upset, but again as I’ve stated several times before, “The confines of the minitruck do not leave much room for dangerous designs.” On top of that, I’m sure not many of you have bucket loads of horsepower and if you do it’s not getting to the ground. I’m sure there are more than a few of you that noticed that once you had the reverse 4-link installed it got easier to do a burn out. Another saving grace of the reverse orientation is that roll-steer is almost always roll-understeer making the truck feel stable in corners. Also the pinion angle, when properly designed, is kept well within working limits. The point is, the reverse 4-link works. It holds the truck off of the ground and keeps the rearend under the truck.The 2-forward 2-reverse works because it does nothing, it doesn’t help, it doesn’t hurt… nothing. The biggest issue with the 2-forward 2-reverse is pinion-angle. It’s a little tough to calculate and doesn’t offer much travel because of it. On an odd note, this system doesn’t move much front to rear, as it runs through its travel.I have never seen the reverse 4-link or the 2-forward 2-reverse type 4-link used anywhere else except the minitruckin industry. Bob Grant called me the other day and challenged me to figure out how to build a rear suspension that doesn’t move front to back in the wheel-well. I immediately thought “A-arm” then he clarified “straight axle.” I then promptly said that it couldn’t be done. Then he told me his idea, it would work, but if the rearend doesn’t follow a set point in space (instant center) it will not add or subtract anything to the cause of traction.I hope you were able to get through this and decipher the cryptic writings about the black magic of link-type suspension.

I'll be the first to say people worry about "engineering" way way too much in mini trucks, but having a solid basis in suspension design is a good thing to have. I know we are only building 90hp trucks most times but we are driving 90hp trucks that see speeds of 70mph+. If something is built wrong, and in turn causes parts to fail...its a huge problem if your on the highway next to someone.